The present invention relates to a method of detecting and distinguish impurities (e.g. metal materials, fibers, voids and ambers) in molten resin by passing light from a light source through the molten resin such as molten polyethylene.
When extruding an insulating coating of a polyethylene on a power cable or forming an extrusion-molded joint of polyethylene at joint portions of the cable, the cable may suffer electrical troubles such as electrical breakdown if the polyethylene contains impurities. Thus, it is necessary to inspect the molten polyethylene to measure the number and size of the impurities.
Also, since such molten polyethylene contains a crosslinking agent, if the temperature of the molten polyethylene is not kept uniform, a runaway crosslinking reaction, known as "ambers", may occur. Thus, the detection and measurement of impurities has to be carried out without changing the temperature and flow rate of the molten polyethylene.
FIG. 6 shows one conventional method of detecting impurities in a molten polyethylene. As shown in the figure, a pipe through which molten polyethylene 32 flows is partially formed as a glass pipe 31. A laser beam is emitted from a light source 33. If there exist impurities in the polyethylene, the laser beam is scattered by them. Thus, by catching the scattered light, one can detect that there are impurities and measure their size.
The arrangement of this figure includes a reflecting mirror 34, a lens 35, a light detector 36 and an oscillograph 37.
With this conventional method, it is possible to detect very small impurities on the order of several micrometers with high sensitivity. But this method has one drawback in that the diameter of the impurities cannot be measured with a high accuracy or cannot be measured at all if the impurities have diameters substantially larger than the wavelength of the laser beam (say, several tens of micrometers) because the strength and direction of scattered light vary according to the shape and surface condition of the impurities.
FIG. 7 shows another conventional method for detecting impurities in a molten polyethylene. As shown in the figure, a glass pipe 41 is in the shape of a sheet and is located within the focal depth of a CCD camera 43. The images of impurities in a molten polyethylene 42 that pass through the pipe are picked up in the form of camera images to detect their shades.
In this method, if it is desired to detect minute impurities, it is necessary to use a lens having a long focal distance. A high magnification means a shallow (short) focal depth. Thus, the passage through which molten polyethylene flows has to be sufficiently wide and thin. The polyethylene is thus more likely to suffer from excessive crosslinking reaction, that is, "ambers".
Further, since the molten polyethylene flows through such a passage at a high speed, image processing has to be carried out at an extremely high speed in order to inspect all of the polyethylene. Moreover, since a glass pipe large enough to cover the entire field of view of the camera is needed, it is necessary to heat the pipe to keep the temperature of the molten polyethylene constant.